This term, we will be studying Zadie Smith

Time was when A-level students had to wade through Chaucer, Shakespeare or DH Lawrence. Now contemporary authors are an integral part of the syllabus. But who chooses which books become set texts - and why are the publishing houses so interested? John Mullan reports

Now that the annual Man Booker steeplechase is out of the way, it is worth considering a different, unnoticed guarantee to an author that his or her book might outlive them. It is something more influential than prizes, or current popularity, or the enthusiasm of critics. All over Britain, tens of thousand of teenagers have begun working their way through books that have been chosen by exam boards as the best examples of contemporary literature. Anyone who has done Eng Lit A-level will know how these books - even the necessary "quotes" from these books - can become the ones you remember for the rest of your life. No author can foresee the judgment of posterity, but there is one certain way of extending the lifespan of one's literary creations: become a set text.

Contemporary writers never used to feature on A-level syllabuses. For years, the nearest most candidates got to a living author were the poems that an elderly TS Eliot or WH Auden had published decades earlier. Even by the end of the 1970s, the most up-to-date fiction studied might be one of the novels published by William Golding in the 1950s. Nowadays things are different. This summer candidates were being examined on Zadie Smith, Julian Barnes and Louis de Bernières. Next summer it will be AS Byatt's Possession and Michael Frayn's Spies.

These writers will notice what publishers call a "spike" in their sales, but they are not the secondary-education top guns. The big two novelists are Margaret Atwood and Ian McEwan. Anyone who has interviewed sixth-formers for places on university courses over the past decade will know that Atwood's The Handmaid's Tale has been studied as much as Pride and Prejudice (and a great deal more than any other 19th-century novel). McEwan's Enduring Love has long jostled with it for top slot (and was still being set this year by AQA, the most popular exam board with British teachers).

Atwood's Cat's Eye used to be a set book, and even as The Handmaid's Tale slides off syllabuses, her more recent Alias Grace and Oryx and Crake have appeared on course lists to make up for any lost royalties. McEwan's The Child in Time has been a set text for a couple of examination boards for a decade, and Atonement (a set book for the OCR exam board since last year) looks set for a long run.

Atwood and McEwan are accomplished novelists, of course, but their popularity with exam setters is disproportionate. It is hard not to think that the political tendency of Atwood's The Handmaid's Tale has had something to do with the spell it has cast over examiners. The novel is set in a poisoned future where most women are sterile and the "handmaids" have to provide children for a patriarchal society governed by Old Testament laws. A feminist updating of Nineteen Eighty-Four, but with an optimistic epilogue, its depiction of a perverted Christian fundamentalism surely helped it win its place.

The AQA spokesperson told me that a text had to "sustain successful questions setting", which seemed to mean that you had to be able to ask plenty of different questions about it from year to year. McEwan's boon to the A-level teacher and examiner is the presence of exam-friendly "themes" in his novels. (This is just the reason that EM Forster's ideas-based novels were once such popular examination fare.) The Child in Time opens with a beautifully written account of a man losing his three-year-old child in a supermarket - and never finding her again. Only the sharpest teenage literary critic would be able to analyse why it carries such conviction. But then the novel becomes a rumination on the satisfactions of paternity. A sub-plot explores the illusory attractions of childishness by telling of a Thatcherite MP who regresses into boyhood and ends up living in a treehouse. Designedly it connects the psychological theme to a political one. You can see the essay titles as you read.

The processes by which set texts are chosen by exam boards are obscure, but shape many readers' sense of what Eng Lit is. How are the choices made? Teachers are able to select from the annual menu of books, but the origins of this menu are as mysterious to them as to any candidate. The Qualifications and Curriculum Authority (QCA), the government body that regulates the exam boards, offers the blandest guidance. "The texts read should be of sufficient substance and quality to merit serious consideration, and should have been originally written in English."

Finding out anything directly from the exam boards is difficult. The actual decisions as to set books are arrived at, as far as one can tell, by "senior examiners" (the best description of the decision-making group that I could obtain from any of the organisations) "after wide consultation". Who knows what this means?

Publishers would love some insight, for the decisions have important commercial consequences. Each summer, more than 80,000 candidates sit English A-level. Every one of them will need a copy of each of the works being studied. Most candidates (or their parents) will buy a copy of each set text (so it has to be a paperback). The head of English at a private day school told me that these purchases were taken for granted. An English teacher at a London comprehensive said her school bought copies of the set texts in bulk (at a discount) and then insisted on the pupils buying them off the school (with the discount passed on).

It is hard to tell how many of the hundreds of thousands of copies of The Handmaid's Tale sold in Britain have been bought by A-level students, but the number must be in six figures. Any author (and publisher) would be grateful for A-level status, but for poets the difference between being a set text and not being so can be life or death.

Carol Ann Duffy has plenty of admirers outside staffrooms, but her status as the most widely read British poet has been achieved through school syllabuses. She is currently being set by AQA, the Welsh Joint Education Committee and Edexcel. The royalty repercussions are considerable. Duffy's collection The World's Wife sold more than 5,000 copies in its first year (a bestseller by poetry standards); four years later, its sales, instead of diminishing, almost doubled: it had become an A-level set text. As long as it stays on the syllabus, it will go on selling 5,000 to 10,000 copies a year.

Among playwrights, the boost to sales is also proportionately enormous. For the past 10 or 15 years, the most widely read play in English not written by Shakespeare has been Translations, by the Northern Irish playwright Brian Friel. First staged in 1980, it has been on the syllabuses of all the exam boards since the 1990s and must cumulatively have been studied by hundreds of thousands of candidates. The play's regular annual sales of about 20,000 copies are almost entirely exam-generated. These sales figures seem a useful estimate of the commercial benefit a semi-permanent presence on A-level reading lists can bring.

Why Translations? Set in Donegal in the 19th century, it features the attempts of British soldiers to foist English translations of the ancient place-names on to the Irish-speaking populace. It includes the impossible love affair of a British officer and a local girl. It is a drama of colonialism, and of the colonisation of language, that has perfectly met somebody's political agenda. Friel's more recent exploration of colonialism, Making History, based on the rebellion of Hugh O'Neill, Earl of Tyrone, against the forces of Elizabeth I, has now taken its place on the AQA list of options and is one of this year's set plays for Northern Ireland's A-level candidates.

Political choices have been explicit since the 1980s. Then the whiteness of A-level authors was challenged by the London University board, which introduced "African and the Caribbean" as an option. Writers such as Chinua Achebe, Ngugi Wa Thiong'o and Wole Soyinka suddenly joined Thomas Hardy, Joseph Conrad and EM Forster. Nowadays OCR, the descendant of the old Oxford and Cambridge exam boards, may have a reputation for being "traditional", but it has an option on "post-colonial literature". This has featured Arundhati Roy's The God of Small Things and Michael Ondaatje's The English Patient, and now has Zadie Smith's White Teeth.

War - or rather, anti-war - has also become popular, leading to the choice of Louis de Bernières' Captain Corelli's Mandolin (which has had its swansong for AQA, but is still in the reckoning for OCR) and Sebastian Faulks's Birdsong (in the A-level canon despite failing to win over the critics). The first world war has long been big at A-level and is a mandatory topic for AQA candidates. Faulks is safe territory where more literary work can be dangerous. Salman Rushdie was once edging into the classroom, and Midnight's Children was famously voted the Booker of Bookers, but his fiction now seems far too incendiary a prospect.

A Booker winner can have its critical status cemented by examiners in the manner of Kazuo Ishiguro's The Remains of the Day, which has been set for more than a decade, and is still going strong if you are a candidate for the Northern Irish exam board (CCEA). You sense his nearly-Booker of last year, Never Let Me Go, is destined to be a set text. It has the winning combination of apparent simplicity (the narrator has a limited vocabulary and intellectual horizon) and real complexity (the better the reader, the more is to be inferred). In contrast, the winner of last year's Booker, John Banville, has left A-level setters cold. Too many allusions and too strange a vocabulary, too complete an indulgence in "style".

Choices within American fiction seem peculiarly ideological. The ponderously worthy historical novel Cold Mountain (with back-up DVD of the Hollywood version) is an OCR set text this year. More here on the costs of war. But can you imagine examiners recommending The Corrections, featuring the sexual calamities of the Lambert family? And as for the greatest living American novelist, Philip Roth ... Somehow you know that he is going to be kept well away from young and impressionable readers.

Only those who make these decisions can know how they are influenced by ideological pressures and sensitivities, and how this might lead to exclusion as well as inclusion. Chaucerian bawdy (The Miller's Tale) and Shakespearean seaminess (try Measure for Measure) are apparently safe, but contemporary sex is likely to be controversial. Alan Hollinghurst's The Line of Beauty may have won the Booker prize and made it to BBC1, but will it ever be a set text? Sarah Waters may achieve critical admiration and big sales, but are examiners ready for explicit lesbianism?

Martin Amis's Money, which would feature on many critics' lists of the best British novels of the past 30 years, appears unsettable: pornography, masturbation, misogyny, you name it. Yet it is the satirical gusto that really scuppers its chances, for sex, even nasty sex, is not itself a reason for excluding a book from the schoolroom. Toni Morrison's Beloved, an OCR set book until 2005, features a variety of sexual violence unmatched in serious fiction of recent years. Similarly, Alice Walker's The Color Purple, written in letters by its sexually abused young heroine, was still going strong in two of this year's A-level syllabuses. But then tales of oppression, valiantly overcome (Wild Swans and Oranges Are Not the Only Fruit are further examples) are always favourites with A-level setters.

Every generation has its odd tastes, and the set texts of the past include choices that now seem instructively weird. The first London University A-level in 1953 did have a living author: John Masefield. The work fearlessly chosen was Reynard the Fox, a narrative poem celebrating fox-hunting. In an age when female writers usually meant Jane Austen or George Eliot, Vita Sackville-West's long, dewy-eyed poem about the English countryside, The Land, unaccountably turned up too. For the first generation of candidates, George Bernard Shaw, with all his undigested "ideas" to write about, was inescapable. Some of our age's predilections might eventually seem as unconvincing. BBC journalist Fergal Keane may be a good egg and a fearless reporter, but his sentiment-squeezing memoir Letter to Daniel (an OCR set text since last year) looks destined to tell a future age of our culture's hunger for righteous sensitivity.

Books swim down the gutter of time, as Laurence Sterne put it, or they don't. Posterity, they say, is the final arbiter of literary immortality, and even examiners could not keep John Masefield afloat. So it should be. But it is not only the votes of readers that make the canon, it is also the setters of exam texts. Those nameless - no doubt conscientious - men and women can give posterity a very big nudge.